Stronghold Assault is fine, and in my opinion it can give many armies an answer to fliers that they might not have otherwise.
There are two problems with it...
1. It makes gunlines even stronger by adding more big guns and a place to give better cover for gunline models. This on its own isn't terrible, but it promotes a style of gameplay that isn't very interesting or fun.
2. People tend to say Stronghold Assault in the same breath as Escalation, and Escalation has a lot more hate behind it than SA does. They came out side by side, SA as a small pill to swallow, but Escalation as a huge pill to swallow. It was just a bit too much at once. I think if SA came out a few months before Escalation, we'd see more fortifications in lists.
That said, the nice thing is that you only get one fortification, so it's not like you're overpowering your gunline by giving every Fire Warrior squad or Imperial Guard platoon even more strength, and many of the fortifications have limited carrying capacity, fire points, or don't have either and require models be dedicated to firing those weapons, which is pretty much like giving a model in a unit an extra heavy weapon that can never move. That's pretty balanced.
A lot of players in my local meta don't seem to get the idea that fortifications CAN be destroyed, especially by things like Melta, and that grenades DO exist and can do FANTASTIC damage to models inside fortifications, along with things like flamers. So, benefits are balanced by detriments.
I'd like to see more lists with fortifications, but the points you spend into them take away from points you could be putting into your flying circus, or Heldrakes, or whatever Deathstar you're running, so competitive players tend not to be interested.